Global climate change has sparked initiatives to reduce emissions of acidic gases, such as carbon dioxide (CO2). Removal of acidic gases by absorption/stripping is a commercially promising technology, as it is well suited to sequester CO2. CO2 emissions may be produced by a variety of different processes, such as the gas stream produced by coal-fired power plants and fossil fuel powered vehicles. The removal of CO2 from such gaseous streams can be an expensive process, potentially increasing the cost to produce electricity by 50% or more. Therefore, technology improvements to reduce the costs associated with the removal of CO2 are highly desirable.
Gas absorption is a process in which soluble components of a gas mixture are dissolved in a liquid. Stripping is essentially the inverse of absorption, as it involves the transfer of volatile components from a liquid mixture into a gas. In a typical CO2 removal process, absorption is used to remove CO2 from a combustion gas, and stripping is subsequently used to regenerate the solvent and capture the CO2 contained in the solvent. Once CO2 is removed from combustion gases and other gases, the CO2 can be captured and compressed for use in a number of applications, including sequestration, production of methanol, and tertiary oil recovery.
The conventional method of using absorption/stripping processes to remove CO2 from gaseous streams is described in U.S. Pat. No. 4,384,875, which is incorporated herein by reference. This process is referred to herein as “simple stripping.” In the absorption stage, the gas to be treated, containing the CO2 to be removed, is placed in contact, in an absorber, with the chosen absorbent (i.e., “lean solvent”) under relatively high pressure and relatively low temperature such that the absorbent solution removes virtually all the CO2. The purified gas emerges at the top of the absorber and, if necessary, it is then directed towards a scrubber employing sodium hydroxide, in which the last traces of CO2 are removed. At the bottom of the absorber, the absorbent solution containing CO2 (also called “rich solvent”) is drawn off and subjected to a stripping process to free it of the CO2 and regenerate its absorbent properties. Other methods of using absorption/stripping process to remove CO2 from gaseous stream are described in U.S. Patent Application Publication No. 2011/0171093 and U.S. Pat. No. 7,938,887, the entireties of which are hereby incorporated by reference.
To effect the regeneration of the absorbent solution, the rich solvent drawn off from the bottom of the absorber is introduced into the upper half of a stripper, and the rich solvent is maintained at its boiling point under pressure in the stripper. The heat necessary for maintaining the boiling point of the rich solvent in the stripper is accomplished by a reboiler. In particular, the absorbent solution contained in the stripper is reboiled by indirect heat exchange between part of the solution to be regenerated located in the lower half of the stripper and a hot fluid at appropriate temperature, generally saturated water vapor. In the course of regeneration, the CO2 contained in the rich solvent is released and stripped by the vapors of the absorbent solution. Vapor containing the stripped CO2 emerges at the top of the stripper and is passed through a condenser system, which returns to the stripper the liquid phase resulting from the condensation of the vapors of the absorbent solution. At the bottom of the stripper, the hot regenerated absorbent solution (also called “lean solvent”) is drawn off and recycled to the absorber after having used part of the heat content of the solution to heat, by indirect heat exchange, the rich solvent to be regenerated, before its introduction into the stripper.
A major disadvantage of such prior systems is the large steam consumption of the stripper. A relatively large amount of wasted heat is generated from such systems. Another disadvantage of prior systems is the use of a reboiler, which can significantly increase the cost to perform the stripping if heat recovery means are not employed.